[PATCH] Fix memory order issue inside pv spinlock

["Yang, Xiaowei" <xiaowei.yang@intel.com>]

barrier() can't prevent reads after it not being reordered with older writes to 
different locates before it. Because of it, I can't bring up > 4 HVM guests on 
one SMP machine. Use mb() instead.

Signed-off-by: Yang Xiaowei <xiaowei.yang@intel.com>

diff --git a/arch/x86/xen/spinlock.c b/arch/x86/xen/spinlock.c
index 5601506..9dee5f8 100644
--- a/arch/x86/xen/spinlock.c
+++ b/arch/x86/xen/spinlock.c
@@ -324,7 +325,7 @@ static void xen_spin_unlock(struct raw_spinlock *lock)
         xl->lock = 0;           /* release lock */

         /* make sure unlock happens before kick */
-       barrier();
+       mb();

         if (unlikely(xl->spinners))
                 xen_spin_unlock_slow(xl);

Thanks,
Xiaowei